Here are Panman's instructions on how to clean and season cast iron, including how to remove rust.
I have not used his instructions on rust cleaning, but his seasoning instructions seem to work.
http://panman.com/how-to-clean-season-cast-iron/
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Just for reference, here is Lodge's: https://www.lodgecastiron.com/discover/cleaning-and-care/cast-iron/all-about-seasoning